The present invention relates to a blend of melt processable wholly aromatic polyesters. The characteristics and properties of such a blend differ significantly from those expected in light of observations of the properties of ordinary mixtures and of most polymeric blends.
When a blend or mixture is prepared from two or more ordinary, non-polymeric materials, a random distribution of the molecules of the components is obtained. This random distribution provides complete mixing without the formation of groups or clusters of the molecules of any one component. Such a mixture is expected to follow the "Rule of Mixtures." The Rule of Mixtures predicts the numerical values of properties of a blend to be the weighted average of the numerical values of the properties of the components.
A discussion of the Rule of Mixtures can be found in Predicting the Properties of Mixtures: Mixture Rules in Science and Engineering, by Lawrence E. Nielsen, Marcel Dekker Inc. (New York).
Further information with regard to the Rule of Mixtures can be found on pages 395, 436, 465, 492, and 500 of Volume 2 of Mechanical Properties of Polymers and Composites, by Lawrence E. Nielsen, Marcel Dekker Inc. (New York: 1974). As stated therein, mixtures of a polymer matrix with a fibrous reinforcing agent, a ribbon-shaped filler, or a rod-shaped filler are known to follow the Rule of Mixtures. The above-cited reference further discloses that mixtures of phase inverted isotropic interpenetrating polymer networks, such as a phase inverted network of polystyrene and polybutadiene, are also known to follow the Rule of Mixtures.
Mixtures of chemically distinct polymeric materials have been found to deviate from the behavior of ordinary mixtures as characterized by the Rule of Mixtures. The sheer size of polymeric chains restricts mixing of the components and leads to the formation of domains or clusters of molecules of the individual components. Thus, it can be said that most chemically distinct polymeric materials tend to be incompatible in mixtures, exhibiting a tendency to separate into phases. There exists a boundary between the domains of the component polymers, and articles made from such a blend would be expected to exhibit failure at the boundary when placed under stress. In general, then, the mechanical properties of the product are commonly reduced rather than enhanced. Specific properties which may be thus affected include tensile strength, tensile modulus, flexural strength, flexural modulus, and impact strength.
Some polymeric materials, such as most wholly aromatic polyesters, exhibit an ordered structure in at least some regions of the polymer. This order can exist in one, two, or three dimensions. The inclusion of such polymers in blends leads to an increased tendency of the blends to separate into phases. This is due to the fact that the order found in certain regions of the polymer causes a fairly sharp boundary between the domains of the molecules of the component polymers. Thus, blends including such polymers would be expected to exhibit a significant reduction in mechanical properties. Accordingly, there has been little impetus to form such blends, particularly for use in applications where mechanical properties are of importance.
U.K. Published Patent Application No. 2,008,598 discloses a polymer composition comprising 20 percent or less, based upon the total weight of polymeric material, of a first rigid polymeric material with the balance being a second polymeric material composed substantially of flexible molecular chains. The first polymeric material is dispersed in the second polymeric material in a microscopic region of 1 .mu.m. or less. Foreign counterparts of this application include Japan No. 54065747, French No. 2407956, and West German No. 2847782. This application does not disclose blends of two rigid polymeric materials, such as two wholly aromatic polyesters.
It is therefore an object of the present invention to provide a blend of wholly aromatic polyesters which exhibits satisfactory mechanical properties, such as tensile strength, tensile modulus, flexural strength, flexural modulus, impact strength, and heat deflection temperature.
It is also an object of the present invention to provide a blend of wholly aromatic polyesters which exhibits, within at least some compositional ranges, mechanical properties, such as flexural strength, which are improved over those of each component alone.
It is also an object of the present invention to provide a blend of wholly aromatic polyesters which exhibits no significant reduction in mechanical properties, such as tensile and flexural strengths, as compared to the weighted average of the mechanical properties of the individual components.
It is also an object of the present invention to provide a blend of wholly aromatic polyesters which exhibits a high degree of anisotropy and shear sensitivity in the melt.
These and other objects as well as the scope, nature, and utilization of the present invention will be apparent from the following description and appended claims.